Core–Shell FeCoNi Nanoparticles in Carbon Aerogels for Highly Efficient Electromagnetic Wave Absorption

Utilizing the multiple magnetic resonance properties of polymetallic alloys and considering the high electrical conductivity, porous structure, and high specific surface area of traditional phenolic-aldehyde-based carbon aerogels (CAs), ternary FeCoNi alloy nanoparticle-doped CAs (FeCoNi-CAs) with impressive core–shell structure were prepared via a simplified sol–gel method using 1,2-epoxypropane as a catalyst. The composition, conductivity, morphology, textural characteristics, electromagnetic wave absorption (EMA) properties and mechanisms, and radar cross-section (RCS) values of FeCoNi-CAs were first investigated in detail by adjusting the molar ratio of Fe, Co, and Ni elements. The results revealed that the optimum EMA properties occur at the molar ratio of Fe/Co/Ni = 4:1:5, and its corresponding Fe₄Co₁Ni₅-CAs have a maximum effective absorption bandwidth (EAB_max) of 6.21 GHz (10.43–16.64 GHz) at a matching thickness of only 1.91 mm. Meanwhile, it possesses a minimum reflection loss (RL_min) of −54.9 dB and an EAB of 5.61 GHz (11.11–16.72 GHz) at the same matching thickness of only 1.87 mm in the medium- and high-frequency regions. The Fe₄Co₁Ni₅-CAs also show the EAB_max of 2.38 GHz (matching thickness: only 3.4 mm, 5.59–7.97 GHz) in the low-frequency region. Further analysis demonstrates that the excellent EMA performance of Fe₄Co₁Ni₅-CAs is mainly ascribed to its higher conductivity stemming from its unique Fe/Co/Ni ratio. Not only does this ratio promote the dielectric loss, but simultaneously, it heightens the magnetic loss. This, in turn, intensifies the combined influence of both dielectric and magnetic losses, giving rise to excellent impedance matching properties. This study could offer a catalyst and a more feasible way to synthesize polymetallic nanoparticle-doped CAs featuring diverse microstructures and superior EMA characteristics.